The present invention is concerned with cationic ion-exchange resins, particularly in the form of membranes, preferably partially or completely fluorinated, their applications, in particular in electrochemical applications such as fuel cells, alkali-chloride processes, electrodialysis, ozone production, as well as any other application related to the dissociation of anionic centers linked to the membrane, such as heterogeneous catalysis in organic chemistry.
Because of their chemical inertia, ion-exchange membranes partially or completely fluorinated are usually chosen for alkali-chloride processes or fuel cells consuming hydrogen or methanol. Such membranes are commercially available under trade names like Nafion(trademark), Flemion(trademark), Dow(trademark). Other similar membranes are proposed by Ballard Inc. in application WO 97/25369 that describes copolymers of tetrafluoroethylene and perfluorovinylethers or trifluorovinylstyrene. The active monomers from which these copolymers are obtained bear chemical functions that are the precursors of ionic groups of the sulfonate or carboxylate type. Example of such precursors are: 
wherein
X is F, Cl or CF3;
n is 0 to 10 inclusively; and
p is 1 or 2.
Aromatic polymers of the polyimide or sulfonated polyether sulfone type have also been considered, for example: 
Once obtained, the copolymer containing the above precursors is molded, for example in the form of sheets, and converted into an ionic form through hydrolysis, to give species of the sulfonate or carboxylate type. The cation associated to the sulfonate and carboxylate anion include the proton, an alkali metal cation (Li+, Na+, K+); an alkaline-earth metal cation (Mg2+, Ca2+, Ba2+); a transition metal cation (Zn2+, Cu2+); Al3+; Fe3+; a rare earth cation (Sc3+, Y3+, La3+); an organic cation of the xe2x80x9coniumxe2x80x9d type, such as oxonium, ammonium, pyridinium, guanidinium, amidinium, sulfonium, phosphonium, these organic cations being optionally substituted by one or more organic radicals; an organometallic cation such as metallocenium, arene-metallocenium, alkylsilyl, alkylgermanyl or alkyltin.
Such membranes suffer from many important disadvantages.
A) Although the copolymers forming the membrane are insoluble in their ionic form, the membrane does not have a good dimensional stability and swells significantly in water or polar solvents. These copolymers form inverted micellia only when heated at high temperatures in a specific mixture water-alcohol that, after evaporation, allows the production of a film. However, this film regenerated in the solid form does not have good mechanical properties.
B) Tetrafluoroethylene (TFE) is a hazardous product to handle, because its polymerisation is performed under pressure and can cause uncontrolled reactions, particularly in the presence of oxygen. Because of the difference of boiling points between the two monomers forming the copolymer, as well as their polarity difference, it is difficult to obtain a statistical copolymer corresponding to the addition rate of each monomer.
C) The ionic groups in high concentration on the chain have a tendency to cause solubilisation of the copolymer. To prevent this phenomenon, the concentration of ionic groups is kept fairly low by adding an important molar fraction of TFE monomers and/or by increasing the secondary chains length (n greater than 1), with the end result that the concentration of the exchangeable ion groups are less than 1 milliequivalent per gram. Consequently, the conductivity is relatively low and highly sensitive to the water content of the membrane, particularly when the latter is acidified for applications in a fuel cell.
D) The penetration of methanol and oxygen through the membrane is high, because the perfluorocarbonated portion of the polymer allows an easy diffusion of the molecular species, which will chemically react at the opposite electrode and cause a loss of faradic efficiency, mainly in methanol fuel cells.
Non-fluorinated systems like sulfonated polyimides or sulfonated polyether sulfones have the same drawbacks because one must compromise between the charged density, and thus the conductivity, and the solubility or excessive swelling.
The present invention concerns a sulfonated polymer comprising a fraction or all the sulfonyl groups cross-linked, and wherein at least one fraction of the cross-linking bonds bear an ionic charge. More specifically, the cross-linking bonds are of the type:
Pxe2x80x94SO2xe2x80x94Yxe2x88x92(M+)xe2x80x94SO2xe2x80x94Pxe2x80x2
Pxe2x80x94SO2(M+)Yxe2x88x92SO2xe2x80x94(Qxe2x80x94SO2)rYxe2x88x92(M+)SO2xe2x80x94Pxe2x80x2
wherein
P and Pxe2x80x2 are the same or different and are part of a polymeric chain;
Y comprises N or CR wherein R comprises H, CN, F, SO2R3, C1-20 alkyl substituted or unsubstituted; C1-20 aryl substituted or unsubstituted; C1-20 alkylene substituted or unsubstituted, wherein the substituent comprises one or more halogen, and wherein the chain comprises one or more substituent F, SO2R, aza, oxa, thia ou dioxathia;
R3 comprises F, C1-20 alkyl substituted or unsubstituted; C1-20 aryl substituted or unsubstituted; C1-20 alkylene substituted or unsubstituted, wherein the substituent comprises one or more halogens;
M+ comprises an inorganic or organic cation;
Q comprises a divalent radical C1-20 alkyl, C1-20 oxaalkyl, C1-20 azaalkyl, C1-20 thiaalkyl, C1-20 aryl or C1-20 alkylaryl, each being optionally substituted by one or more halogens, and wherein the chain comprises one or more substituents oxa, aza or thia; and
r is 0 or 1.
In a preferred embodiment, M+ comprises the proton, a metal cation, an organometallic cation or an organic cation, the latter 2 optionally substituted with one or more organic radicals comprising:
a proton, an alkyl, an alkenyl, an oxaalkyl, an oxaalkenyl, an azaalkyl, an azaalkenyl, a thiaalkyl, a thiaalkenyl, a dialkylazo, a silaalkyl optionally hydrolysable, a silaalkenyl optionally hydrolysable, each being straight, branched or cyclic and comprising from 1 to 18 carbon atoms;
a cyclic or heterocyclic aliphatic radical comprising from 4 to 26 carbon atoms optionally comprising at least one lateral chain comprising one or more heteroatoms such as nitrogen, oxygen or sulfur;
an aryl, an arylalkyl, an alkylaryl and an alkenylaryl of from 5 to 26 carbon atoms optionally comprising one or more heteroatoms in the aromatic nucleus or in a substituent.
The metal preferably comprises an alkaline metal, an alkaline-earth metal, a rare earth or a transition metal; the organometallic cation comprises a metallocenium, an arene-metallocenium, an alkylsilyl, an alkylgermanyl or an alkyltin, and the organic cation comprises Rxe2x80x3O+ (onium), NRxe2x80x3+ (ammonium), Rxe2x80x3C(NHRxe2x80x3)2+ (amidinium), C(NHRxe2x80x3)3+ (guanidinium), C5Rxe2x80x3N+ (pyridinium), C3Rxe2x80x3N2+ (imidazolium), C2Rxe2x80x3N3 (triazolium), C3Rxe2x80x3N2+ (imidazolinium), SRxe2x80x3+ (sulfonium), PRxe2x80x3+ (phosphonium), IRxe2x80x3+ (iodonium), (C6Rxe2x80x3)3C+ (carbonium), wherein Rxe2x80x3 is defined as an organic radical as defined above, and when an organic cation comprises at least two radicals Rxe2x80x3 other than H, these radicals can form together a cycle, aromatic or not, eventually containing the center bearing the cationing charge.
In a further preferred embodiment, the divalent radical Q and the sulfonated polymer are partially or completely fluorinated.
The present invention further comprises a process for cross-linking sulfonyl groups of a sulfonated polymer wherein at least a fraction of the cross-linking bonds bear an ionic charge, the process comprising mixing the polymer with a cross-linking agent allowing the reaction between 2 sulfonyl groups from adjacent polymeric chains, to form the said cross-linking bonds. Preferred cross-linking agents are of formula
(M+)A2Yxe2x88x92; 
(M+)AYxe2x88x92SO2Yxe2x88x92A(Mxe2x88x92); 
(Mxe2x88x92)AYxe2x88x92SO2QYxe2x88x92A(M+) 
wherein Y, Q and M are as defined above, and A comprises Si(Rxe2x80x2)3, Ge(Rxe2x80x2)3 or Sn(Rxe2x80x2)3 wherein Rxe2x80x2 is C1-18 alkyl.
It is well known that perfluorinated polymers cannot usually be cross-linked by conventional techniques used for non-fluorinated polymers because of the easy elimination of the fluoride ion and the steric hindrance of the perfluorinated chains. However, the present invention describes a novel general technique to create cross-links, i.e., bonds, between sulfonyl groups attached to adjacent polymeric chains, including those with a perfluorinated skeleton, for example those derived from monomer (I) and its copolymers: 
Advantageously, the cross-linking can be performed when the polymer is in the form of a non-ionic polymer precursor, but after having been molded in the desired form. The end result is therefore a material having enhanced mechanical resistance. The present invention also concerns the molding of the cross-linked polymer in the form of a membrane or hollow fibers, (hereinafter xe2x80x9cmembranesxe2x80x9d) for use in a fuel cell, a water electolyser, an alkali-chloride process, electrosynthesis, water treatment and ozone production. The use of the cross-linked polymers as catalysts for certain chemical reactions, because of the strong dissociation of the ionic groups introduced by the cross-linking technique and the insolubility of the polymeric chain, are also part of the invention.
The creation of stable cross-links is performed by a reaction between two xe2x80x94SO2Y groups from adjacent polymeric chains. The reaction is initiated by a cross-linking agent, and allows the formation of derivatives of the following forms: 
wherein r, M, Y and Q are as defined above;
A comprises M, Si(Rxe2x80x2)3, Ge(Rxe2x80x2)3 or Sn(Rxe2x80x2)3 wherein Rxe2x80x2 is C1-18 alkyl; and
L comprises a leaving group such as a halogen (F, Cl, Br), an electrophilic heterocyclic N-imidazolyl or N-triazolyl, R2SO3 wherein R2 is an organic radical as defined above.
The cation M+ can itself be solvated or complexed to increase its solubility and/or its reactivity. For example, if M is a proton, the latter can be complexed with the help of a tertiary base having a strong nucleophilic character, such as triethylamine, dimethylaminopyridine, 1,4-diazabicyclo[2.2.2]octane, or in the form of a tertiobutyle radical that easily separates into a proton and CH2xe2x95x90C(CH3)3. If M is a metallic ion, the latter can be solvated by dialkylethers of oligo-ethylene glycols, or methylated oligo-ethylenediamines.
Alternately, the cross-linking agent A2Yxe2x88x92(M+) can be formed in situ in the presence of a strong base, for example an organometallic or a metallic dialkyl amine such as diisopropylamide-lithium reacting on the leaving protons linked to the Y radical in the following manner:
HN[Si(CH3)3]2+C4H9LiC4H10+LiN[Si(CH3)3]2; 
CH2[Si(CH3)3]SO2CF3+2CH3MgClLi2CH4+(MgCl)2C[Si(CH3)3]SO2CF3 
Preferred organometallic cross-linking agents include organo-lithium, organo-magnesium or organo-aluminium, that are also a carbon source when Yxe2x95x90CR, and amides and metallic nitrides as a nitrogen source when Yxe2x95x90N.
An advantage of the present invention is that the cross-linking agents provide negatively charged species that are bound to the sulfonyl groups of the polymers, and used as bridges between adjacent polymeric chains. It is well known that sulfonylimide groups and di- or trisulfonylmethane groups are strong electrolytes in most media, and therefore, the cross-linking reaction, in addition to improving the mechanical properties, does not have any detrimental effect on the conductivity. In fact, the latter is often increased.
The following compounds are preferred cross-linking ionogenes agents, i.e., ionic groups generators, when L is on the polymeric chain: Li3N; C3Al4; [(CH3)3Si]2NLi (or Na or K); NH3+3 DABCO; CF3SO2C[(CH3)3Si][Li(TMEDA)]2; (CH3)3CNH2+3 TEA; NH2SO2NH2+4 TEA; [[(CH3)3Si](Li)N]2SO2; [(TMEDA)(Mg)N]2SO2; CH3Li; (CH3)3Al; NH2Li (or Na or K); [[Si(CH3)3](Li)NSO2]2CF2, [Li[Si(CH3)3]NSO2CF2]2CF2; [(Li)Si(CH3)3NSO2CF2]; and [Li[Si(CH3)3]NSO2CF2CF2]2O, wherein TEA=trixc3xa9thylamine; TMEDA=N,N,Nxe2x80x2Nxe2x80x2 tetramethylethylene diamine and DABCO=1,4-diazabicyclo-[2,2,2,]-octane.
Alternately, the cross-linking, reaction can take place when the Y group is already on the precursor of the polymer, for instance in the case of a substituted amide. In such a case, the general scheme is as follows: 
The following compounds are examples of preferred ionogene cross-linking agents when L is on the reagent: SO2Cl2+3 DABCO; SO2(imidazole)2; [FSO2CF2]2+3 TEA; (ClSO2CF2)CF2+3 DABCO and (FSO2CF2CF1)2O+3 DABCO.
The cross-linking reaction may imply all the sulfonyl groups, or only a fraction thereof. These cross-linking reagents can be added or used according to various techniques well known to those skilled in the art. Advantageously, the polymer is molded in the desired form prior to the cross-linking, for example in the form of a membrane or a hollow fiber, and the material is immerged or covered with a solution of the cross-linking agent in one or more solvent favoring the coupling reaction. Preferred solvents are polyhalocarbons, tetrahydrofuran (THF), glymes, tertiary alkylamides such as dimethylformamide, N-methylpyrrolidone, tetramethylurea and its cyclic analogues, N-alkylimidazoles, and tetraalkylsulfamides. The desired cross-linking degree can be controlled through various factors, such as the time of immersion in the solvent containing the cross-linking agent, the temperature of the solvent, the concentration of the cross-linking agent in the solvent, or a combination thereof. Preferably, these parameters are adjusted to produce the desired properties in a relatively short period of time varying between a few seconds to about ten hours, and the temperatures are chosen to be compatible with the usual solvents, from xe2x88x9210xc2x0 C. to 250xc2x0 C. For comparison purposes, hydrolysis of a Nafion(copyright) membrane takes more than 24 hours for usual thicknesses.
Alternately, a latex of the polymer to be molded is mixed preferably in the presence of fluids that are not solvents, such as ordinary or fluorinated hydrocarbons, with the solid cross-linking agent, and the mixture is heat pressed or calendered. This technique can be applied advantageously to thin membranes, and provides high productivity eventhough it is possible that the membrane be less homogeneous. Reinforcing agents such as fillers, organic or inorganic, like powders, fibers or strands woven or not, can be added to the polymers before the cross-linking reaction to reinforce the structure. Also, agents creating of porosity can be incorporated if necessary to increase the exchange surfaces with external fluids (catalytic purposes).
If only a fraction of the links bridging the polymeric chains are required, the remaining SO2Y groups can be hydrolysed conventionally in the sulfonate form by alkaline hydrolysis. Alternately, in a preferred embodiment, the sulfonate group xe2x80x94SO3xe2x88x92M+ and the non cross-linked groups xe2x80x94SO2NSO2RFxe2x88x92M+ or xe2x80x94SO2C(R)SO2RFxe2x88x92M+ wherein RF comprises an organic radical preferably halogenated, particularly fluorinated, can be obtained in the same conditions as for the cross-linking reactions from non cross-linking ionogene agents such as M[(CH3)3SiO], M[(CH3)3SiNSO2RF] or M[(CH3)3SiC(R)SO2RF], or any other agent capable of introducing xe2x80x94NSO2RF or C(R)SO2RF groups as a replacement for Y. It can be advantageous to treat the membrane sequentially with the cross-linking agent, and then with the non cross-linking ionogene agent. Alternately, the cross-linking agent and in the non cross-linking ionogene agent are mixed and dissolved in a solvent in predetermined concentrations, so that they can react simultaneously.
The cross-linked polymer obtained in accordance with the process of the present invention can be easily separated from the secondary products of the reaction, that are volatile, such as (CH3)3SiF or (CH3)3SiCl. Alternately, the cross-linked polymer can be washed with an appropriate solvent such as water or an organic solvent wherein it is insoluble. Further, conventional techniques well known to those skilled in the art, for example ion exchange or electrophoresis, can be used to change the cation M+ obtained in the cross-linking reaction and/or coming from the non cross-linking ionogene agent, by the desired cation for the final application.
The following examples are provided to illustrate the invention and should not be considered as limiting its scope.